A lithium ion secondary battery has advantages such as a high capacity, a light weight and a long life, and hence it has rapidly prevailed as a power source of a portable electronic apparatus such as a cellular phone, a notebook-size personal computer, a video movie camera or a digital camera. In recent years, a demand for the high capacity of the lithium ion secondary battery is increasing more and more. Heretofore, as a material for a negative electrode of the lithium ion battery, there has been used a carbon material such as sparingly graphitizable carbon or graphite, but the effective capacity of such a carbon material has already been saturated from the viewpoint of an industrial technology, and it is difficult to further heighten the capacity. To solve the problem, it has recently been investigated to use a new negative electrode material of a so-called alloy based negative electrode, for example, a metal such as silicon (Si) or tin (Sn), or a semimetal disclosed in Patent Document 1.
However, it is known that the volume expansion coefficients of Si and Sn during the occlusion of lithium are around 300% and 250%, respectively, whereas that of carbon is 12%. Therefore, each volume of Si and Sn changes 20 times or more as much as that of carbon. Moreover, it has been pointed out that a solid electrolyte interface (SEI) formed on the surface of the negative electrode is broken and repaired each time the electrode noticeably expands and contracts, and in consequence, lithium in the battery is noticeably consumed, which deteriorates cycle characteristics of the battery. Owing to this fact, Si and Sn are not broadly put to practical use, though each of Si and Sn has a very attractive potential ability that its theoretical capacity is about ten times as much as that of carbon. For example, in Patent Document 2, it has been investigated that the shape of the Si-based negative electrode is contrived to alleviate an expansion amount, but only by this contrivance, the deterioration of the cycle characteristics cannot sufficiently be suppressed. On the other hand, when such an alloy based negative electrode material is used, a separator adjacent to this material is directly under its influence. Therefore, the separator is required to have compressive resistance as a function of absorbing or withstanding the expansion and contraction of the material.
In addition to the requirement of the compressive resistance, it is also expected that the separator can keep its basic function which is insulation between positive and negative electrodes, and can prevent the deterioration of a battery performance, for example, the deterioration of the cycle characteristics due to clogging caused by compression.
In Patent Document 3, the strength of the separator is regulated to try the improvement of the characteristics of the battery at a high temperature or the safety of the battery, but the alloy based negative electrode noticeably expands or contracts even at normal temperature, and hence the battery characteristics at normal temperature are not sufficiently improved.    Patent Document 1: U.S. Pat. No. 4,950,566    Patent Document 2: Japanese Patent Application Laid-Open No. 2002-83594    Patent Document 3: Japanese Patent Application Laid-Open No. 2006-134757